The play clock is either 25 seconds or 40 seconds. Here's the new rule:

The offense must begin substituting players before there are 15 seconds left on the play clock. If they do not start to sub before 15 seconds left they give up the opportunity to sub, unless they call a timeout.

If the offense legally begins subs, snapping the ball will be held until there are exactly 5 seconds on the play clock, whether either side is finished subbing or not, allowing the defense to sub but not delay.